The invariably fatal, human brain disease, progressive multifocal leukoencephalopathy (PML), is a significant cause of morbidity and mortality in HIV-seropositive individuals. Although the etiologic agent, JC Virus (JCV) is ubiquitous, the incidence of PML in the HIV-seronegative population is extremely rare. The etiology is believed to be due to a reactivation of a latent JCV infection concomitant with a disorder of the cellular immune response, however the incidence of PML in the HIV- seropositive population is much higher than would be expected solely as a consequence of the cellular immune defect present in AIDS disease. Evidence suggests that the late promotor of JCV, which may be restricting JCV replication in its latent state, is transactivated by HIV tat protein. Unfortunately these studies to date have consisted of JC subgenomic fragments, transactivating protein expressed from strong heterologous promoters, and most significantly, the assays have been performed in a glioblastoma cell line, in which JC Virus does not replicate. The biologically relevant experiments are daunting as intact JC Virus replication in vitro is weak and protracted and the only cell type permissive for JCV is the spongioblast, a fragile, fastidious cell type prepared from primary human fetal glial (PHFG) cells. The object of this proposal is to analyze the molecular interactions between HIV-1 and JCV in a biologically relevant system, i.e., one in which JCV replicates. The assays will be performed in PHFG cultures and HIV will be provided by HIV- infected macrophages. The specific aims of the proposed studies are: 1) To determine whether JCV transactivation occurs during coinfection of PHFG with HIV-1 2) To determine whether JCV transactivation by HIV is mediated by macrophages 3) To discern whether transactivation requires cell-to-cell contact 4) To elucidate the HIV x JCV molecular interaction, specifically: a) To determine whether the only HIV component necessary for the transactivation of JCV is the tat protein itself b) To demonstrate that the tat-determined transactivation of JC Virus is indeed mediated through the TAR-like element on the JC Virus promoter Results from these studies will provide fundamental insight into possible interaction of JCV and HIV and provide the rationale for prophylactic and therapeutic approaches to alleviate PML in AIDS.